android - OpenGL ES 2 球体渲染

Question

我一直致力于在适用于 Android 的 OpenGL ES 2.0 中使用三角形带渲染球体。我遇到了一个问题，当球体旋转时，它似乎在自身上重叠。

我创建顶点列表的代码是

private static final int FLOATS_PER_VERTEX = 5;
private final float[] vertexData;
private final List<DrawCommand> drawList = new ArrayList<>();
private int offset = 0;


private ObjectBuilder(int sizeInVertices)
{
    vertexData = new float[sizeInVertices * FLOATS_PER_VERTEX];
}


private void appendSphere(double radius, int depth)
{
double x, y, z, h, altitude, azimuth;

// Ensure that the depth is between 1 and MAX_DEPTH
int clampDepth = Math.max(1, Math.min(5, depth));

// Calculate the sphere values
int numStrips = (int) Math.pow(2, clampDepth - 1) * 5;

final int numVerticesPerStrip = (int) Math.pow(2, clampDepth) * 3;

double altitudeStepAngle = ONE_TWENTY_DEGREES / Math.pow(2, clampDepth);
double azimuthStepAngle = THREE_SIXTY_DEGREES / numStrips;

// Loop through each strip
for (int i = 0; i < numStrips; i++)
{
    final int startVertex = offset / FLOATS_PER_VERTEX;

    // Calculate the position of the first vertex in the strip
    altitude = NINETY_DEGREES;
    azimuth = i * azimuthStepAngle;

    // Draw the rest of the strip
    for (int j = 0; j < numVerticesPerStrip; j += 2)
    {
        // First point - Vertex.
        y = radius * Math.sin(altitude);
        h = radius * Math.cos(altitude);
        z = h * Math.sin(azimuth);
        x = h * Math.cos(azimuth);
        vertexData[offset++] = (float) x;
        vertexData[offset++] = (float) y;
        vertexData[offset++] = (float) z;

        // First point - Texture.
        vertexData[offset++] = (float) (1 - azimuth / THREE_SIXTY_DEGREES);
        vertexData[offset++] = (float) (1 - (altitude + NINETY_DEGREES) / ONE_EIGHTY_DEGREES);

        // Second point - Vertex.
        altitude -= altitudeStepAngle;
        azimuth -= azimuthStepAngle / 2.0;
        y = radius * Math.sin(altitude);
        h = radius * Math.cos(altitude);
        z = h * Math.sin(azimuth);
        x = h * Math.cos(azimuth);
        vertexData[offset++] = (float) x;
        vertexData[offset++] = (float) y;
        vertexData[offset++] = (float) z;

        // Second point - Texture.
        vertexData[offset++] = (float) (1 - azimuth / THREE_SIXTY_DEGREES);
        vertexData[offset++] = (float) (1 - (altitude + NINETY_DEGREES) / ONE_EIGHTY_DEGREES);

        azimuth += azimuthStepAngle;
    }

    drawList.add(new DrawCommand()
    {
        @Override
        public void draw()
        {
            glDrawArrays(GL_TRIANGLE_STRIP, startVertex, numVerticesPerStrip);
        }
    });
}
}

我的渲染代码将多个观察矩阵和透视矩阵相乘，然后执行以下操作:

positionObjectInScene(0f, 0f, 0f);
textureProgram.useProgram();
textureProgram.setUniforms(modelViewProjectionMatrix, texture);
planet.bindData(textureProgram);
glFrontFace(GL_CW);
planet.draw();

渲染中显然涉及很多不同的部分。不过，我认为问题在于顶点生成。

Answer 1

以下代码 fragment 将为要在 OpenGL-ES 中渲染的球体生成顶点、法线、纹理坐标和顶点索引:

public float[] mVertices;
public float[] mNormals;
public float[] mTexture;
public char[] mIndexes;

// rings defines how many circles exists from the bottom to the top of the sphere
// sectors defines how many vertexes define a single ring
// radius defines the distance of every vertex from the center of the sphere.
public void generateSphereData(int totalRings, int totalSectors, float radius)
    {
        mVertices = new float[totalRings * totalSectors * 3];
        mNormals = new float[totalRings * totalSectors * 3];
        mTexture = new float[totalRings * totalSectors * 2];
        mIndexes = new char[totalRings * totalSectors * 6];

        float R = 1f / (float)(totalRings-1);
        float S = 1f / (float)(totalSectors-1);
        int r, s;

        float x, y, z;
        int vertexIndex = 0, textureIndex = 0, indexIndex = 0, normalIndex = 0;

        for(r = 0; r < totalRings; r++)
        {
            for(s = 0; s < totalSectors; s++)
            {
                y = (float)Math.sin((-Math.PI / 2f) + Math.PI * r * R );
                x = (float)Math.cos(2f * Math.PI * s * S) * (float)Math.sin(Math.PI * r * R );
                z = (float)Math.sin(2f * Math.PI * s * S) * (float)Math.sin(Math.PI * r * R );

                if (mTexture != null)
                {
                    mTexture[textureIndex] = s * S;
                    mTexture[textureIndex + 1] = r * R;

                    textureIndex += 2;
                }

                mVertices[vertexIndex] = x * radius;
                mVertices[vertexIndex + 1] = y * radius;
                mVertices[vertexIndex + 2] = z * radius;

                vertexIndex += 3;

                mNormals[normalIndex] = x;
                mNormals[normalIndex + 1] = y;
                mNormals[normalIndex + 2] = z;

                normalIndex += 3;
            }
        }


        int r1, s1;
        for(r = 0; r < totalRings ; r++)
        {
            for(s = 0; s < totalSectors ; s++)
            {
                r1 = (r + 1 == totalRings) ? 0 : r + 1;
                s1 = (s + 1 == totalSectors) ? 0 : s + 1;

                mIndexes[indexIndex] = (char)(r * totalSectors + s);
                mIndexes[indexIndex + 1] = (char)(r * totalSectors + (s1));
                mIndexes[indexIndex + 2] = (char)((r1) * totalSectors + (s1));

                mIndexes[indexIndex + 3] = (char)((r1) * totalSectors + s);
                mIndexes[indexIndex + 4] = (char)((r1) * totalSectors + (s1));
                mIndexes[indexIndex + 5] = (char)(r * totalSectors + s);
                indexIndex += 6;
            }
        }
    }

根据您的代码结构，您可能需要重构代码以满足您的目的，但一般来说，顶点、法线、纹理坐标和顶点索引将生成一个具有纹理和法线的球体(如果您使用照明或您对法线有任何其他需求)。

您需要为这段代码生成的每个数组创建一个缓冲区，然后按以下方式绑定(bind)它们:

    mVertexBuffer = ByteBuffer.allocateDirect(mVertexArray.length * 4).order(ByteOrder.nativeOrder()).asFloatBuffer();
    mVertexBuffer.put(mVertexArray).position(0);

    mIndexBuffer = ByteBuffer.allocateDirect(mIndexArray.length * 4).order(ByteOrder.nativeOrder()).asCharBuffer();
    mIndexBuffer.put(mIndexArray).position(0);

    mTextureCoordinateBuffer = ByteBuffer.allocateDirect(mTextureCoordinatesArray.length * 4).order(ByteOrder.nativeOrder()).asFloatBuffer();
    mTextureCoordinateBuffer.put(mTextureCoordinatesArray).position(0);

    mNormalBuffer = ByteBuffer.allocateDirect(mNormalArray.length * 4).order(ByteOrder.nativeOrder()).asFloatBuffer();
    mNormalBuffer.put(mNormalArray).position(0);


    vertexBuffer.position(0);
    GLES20.glVertexAttribPointer(getParamId(PARAM_VERTEX_POSITION), 3, GLES20.GL_FLOAT, false, 0, vertexBuffer);
    GLES20.glEnableVertexAttribArray(getParamId(PARAM_VERTEX_POSITION));

    normalBuffer.position(0);
    GLES20.glVertexAttribPointer(getParamId(PARAM_VERTEX_NORMAL), 3, GLES20.GL_FLOAT, false, 0, normalBuffer);
    GLES20.glEnableVertexAttribArray(getParamId(PARAM_VERTEX_NORMAL));

    textureCoordinateBuffer.position(0);
    GLES20.glVertexAttribPointer(getParamId(PARAM_VERTEX_TEXTURE_COORDINATES), 2, GLES20.GL_FLOAT, false, 0, textureCoordinateBuffer);
    GLES20.glEnableVertexAttribArray(getParamId(PARAM_VERTEX_TEXTURE_COORDINATES));

注意:getParamId() 返回 OpenGL 为着色器使用的变量(例如位置、法线和纹理坐标)生成的数字 ID。

完成此操作后，剩下要做的就是使用索引缓冲区进行绘制:

GLES20.glDrawElements(GLES20.GL_TRIANGLES, mIndexArray.length, GLES20.GL_UNSIGNED_SHORT, mIndexBuffer);

希望这能帮助您入门。